In 1965, Airman Second Class Adrian Cronauer (Robin Williams) arrives in Saigon from Crete to work as a DJ for Armed Forces Radio Service. Cronauer is greeted by Private First Class Edward Montesquieu Garlick (Forest Whitaker). Cronauer's irreverence contrasts sharply with many staff members and soon rouses the ire of two of his superiors, Second Lieutenant Steven Hauk (Bruno Kirby) and Sergeant Major Phillip Dickerson (J.T. Walsh). Hauk adheres to strict Army guidelines in terms of humor and music programming, while Dickerson is generally abusive to all enlisted men. However, Brigadier General Taylor (Noble Willingham) and the other DJs quickly grow to like the new man and his brand of comedy. Cronauer's show consists of unpredictable humor segments mixed with news updates (vetted by the station censors) and rock and roll records that are frowned upon by his superiors.

Cronauer meets Trinh (Chintara Sukapatana), a Vietnamese girl, and follows her to an English class. Bribing the teacher to let him take over the job, Cronauer starts instructing the students in the use of American slang. Once class is dismissed, he tries to talk to Trinh but is stopped by her brother Tuan. Instead, Cronauer befriends Tuan and takes him to Jimmy Wah's, the local GI bar, to have drinks with Garlick and the station staff. Two other soldiers, angered at Tuan's presence, initiate a confrontation that Cronauer escalates into a brawl.

Dickerson reprimands Cronauer for this incident, but his broadcasts continue as before. While relaxing in Jimmy Wah's one afternoon, he is pulled outside by Tuan, who says that Trinh likes him and wants to see him, moments before the building explodes, killing two soldiers and leaving Cronauer badly shaken. The cause of the explosion is determined to be a bomb; the news is censored, but Cronauer locks himself in the studio and reports it anyway. Dickerson cuts off the broadcast and Cronauer is suspended. Hauk takes over his shows, but his poor attempts at comedy and insistence on playing polka music lead to a flood of letters and phone calls from servicemen who demand that Cronauer be put back on the air.

In the meantime, Cronauer spends his time drinking and pursuing Trinh, only to be rebuffed at every attempt. At the radio station, Taylor intervenes on Cronauer's behalf, ordering Hauk to reinstate him, but Cronauer refuses to go back to work. Garlick's and Cronauer's vehicle becomes stopped in a congested street amidst a convoy of soldiers from the 1st Infantry Division, who persuade him to do an impromptu "broadcast" for them before they go off to fight. This incident reminds him why his job is important, and he soon returns to the air.

Dickerson then seizes an opportunity to get rid of Cronauer by approving his request to interview soldiers in the field, knowing that the only road into the area, a highway to An Lộc, is controlled by the Viet Cong. Cronauer's and Garlick's Jeep is blown off the road by a mine and they are forced to hide in the jungle from the VC patrols. In Saigon, Tuan learns of their trip after Cronauer fails to show up for English class. He steals a van and drives off after them. After finding them, the van breaks down and they flag down a Marine helicopter to take them back to the city.

At the station, Dickerson confronts Cronauer, declaring he is now off the air for good. His friend Tuan is revealed as a VC operative, known as Phan Duc To, who was responsible for the bombing of Jimmy Wah's and Dickerson has arranged for an honorable discharge. General Taylor arrives and informs Cronauer that, regrettably, he cannot help him since his friendship with Tuan would place the reputation of the US Army at risk. After Taylor leaves, Cronauer asks Dickerson why he engineered his dismissal. Dickerson openly admits his personal dislike for Cronauer. After Cronauer leaves, Taylor casually informs an astonished Dickerson that he is being transferred to Guam, citing Dickerson's vindictive attitude as the reason, but secretly also because he knows what he did but cannot prove it.

Cronauer chases down Tuan after accosting Trinh for her brother's whereabouts. Cronauer loses Tuan in the village, and he decries Tuan's actions against the American forces. Emerging from the shadows, Tuan retorts that the United States military has devastated his family, and that for him that makes the United States the enemy. The next day, on his way to the airport with Garlick, and under MP escort, Cronauer sets up a quick softball game with the students from his English class (fulfilling a promise he made to them), where he gets to say goodbye to Trinh. As he boards the plane, he gives Garlick a taped farewell message; Garlick – taking Cronauer's place as DJ – plays the tape on the air the next morning. It begins with a yell of "Gooooooooooooooooodbye, Vietnam!" and runs through a few of Cronauer's impressions before ending with his wish that everyone will get home safely.

In 1979, Adrian Cronauer pitched a sitcom based on his experiences as an AFRS DJ. TV networks were not interested, however, because they did not see war as comedy material,[2] even though one of the most popular shows at the time was M*A*S*H. Cronauer then revamped his sitcom into a script for a TV movie of the week, which eventually got the attention of Robin Williams.[2] Very little of Cronauer's original treatment remained after writer Mitch Markowitz was brought in.[3] The film was shot in Bangkok, Thailand.[4]

Williams received critical acclaim for his portrayal of Adrian Cronauer in the film, and won multiple awards for his acting.

Good Morning, Vietnam was one of the most successful films of the year, becoming the fourth highest-grossing film of 1987.

The film received outstanding reviews from film critics. The website Rotten Tomatoes, which compiles critical reviews for movies, gave a rating of 89%[5] and the consensus: "A well-calibrated blend of manic comedy and poignant drama, Good Morning, Vietnam offers a captivating look at a wide range of Robin Williams' cinematic gifts." Both Roger Ebert and Gene Siskel of the review show Siskel and Ebert awarded the film "Two Thumbs Up", with Ebert giving the film a four out of four star review in the Chicago Sun-Times.[6]Richard Corliss of Time called the film "the best military comedy since M*A*S*H", and named it one of the best films of the year. Vincent Canby of the New York Times called the film a cinematic "tour de force" and compared Williams' performance to that of "an accomplished actor".[7] Much of the acclaim went to Williams' performance, a role that earned him an Oscar nomination for Best Actor.[8]

John J. Puccio of moviemet.com gave a mixed review, saying that the comedy was hilarious, but that the drama was too "melodramatic".[9]

The soundtrack album was certified platinum in the US.[13] Louis Armstrong's "What A Wonderful World" was released as a single because of the film and reached position 32 on the US Top 40, 20 years after its original release.[14]